The characteristic triad of von Willebrand's disease - prolonged bleeding time, decreased platelet adhesiveness and decreased levels of antihemophilic factor (factor VIII) - will be studied in pigs with this disease. Since the three manifestations of the disease are rarely of the same magnitude in any one animal, efforts to breed pigs with single defects will be attempted. We are fractionating normal porcine plasma in an attempt to discover what corrects each abnormality; in what dose, and whether there is a single corrective substance for all three abnormalities or 3 substances. When plasma was fractionated by DEAE- cellulose or by Sephadex-gel chromatography, or both, the substance which stimulates the characteristic factor VIII overresponse has so far not been dissociated from factor VIII itself. However, the stimulating factor is also found in the same fraction of porcine serum which lacks factor VIII activity. This study has direct relevance to human von Willebrand's disease in that improved diagnosis, treatment and clarification of the inheritance pattern are possible.